This invention relates generally to high frequency power semiconductors, and more specifically to compound semiconductor field effect transistor (FET) structures and applications thereof.
In the area of computer and peripheral power supply applications, there are several factors driving future performance and demand. Such factors include an increase in output power requirements because of higher microprocessor speeds, smaller system size (i.e., reduced circuit board space), lower cost, improved transient response, and lower output voltage ripple (i.e., lower microprocessor operating voltages). Additionally; advancing microprocessor needs, which include decreasing operating voltage and increasing current requirements, will require power conversion devices and circuits that enable highly efficient and tightly regulated power. These devices and circuits must operate at higher frequencies and exhibit enhanced thermal characteristics.
Electronic systems such as computer and peripheral power supply applications often require that multiple dc voltage levels be produced from a single dc voltage source. This conversion is done with electronic circuits such as dc/dc converters. A basic converter circuit is a two-port network having a pair of input terminals and a pair of output terminals. A dc power source is coupled across the two input terminals, and a dc load is coupled across the two output terminals. Within the two-port network, the circuit typically comprises multiple switching devices, appropriate control circuitry, one or more capacitors, and one or more inductors. Typical dc/dc converters include the buck converter, the boost converter, and the buck-boost converter.
An ideal switching device has two states: on and off. In the on state, the ideal device conducts current between two terminals with zero voltage drop across the terminals. In the off state, the ideal device will support any voltage drop across the terminals while conducting zero current between them. A number of different semiconductor devices are used as switches in dc/dc converters, all of which depart from the ideal switching device in one or more ways. Examples of such devices include diodes, bipolar transistors, MOSFETs, silicon controlled rectifiers, and junction field effect transistors.
One problem with typical switching devices is a non-zero voltage between the terminals when in the on state. This results in power dissipation in the switching device, which generates heat and reduces overall circuit efficiency. A second problem involves the dynamic characteristics of the switching device when transitioning between the on state and the off state. Slow switching speeds place a limit on system operating frequency and duty cycle. Each time a device switches between states, a certain amount of energy is lost. The slower a device switches, the greater the energy lost in the circuit. This has significant impact on high frequency and/or high power applications, and contributes significantly to the reduction of overall efficiency of a dc/dc converter.
Most losses in switching power circuits are determined by the physical properties of semiconductor devices. Although silicon based MOSFET devices are a primary choice for many power conversion applications, they have inherent limitations for high frequency applications due to their physical structure. Such limitations include high reverse recovery charge, high gate charge, and high on resistance, which detrimentally impact power dissipation and thermal response characteristics.
Multi-phase dc/dc conversion is a preferred technique for addressing the high current/low operating voltage requirements of present and future microprocessors. In multiphase dc/dc converter architectures, a load is distributed evenly across phase-shifted pulse-width-modulation (PWM) channels and associated switching devices and inductors. This approach spreads power and current dissipation across several power handling devices thereby lowering stress on components. This approach also reduces output ripple on inductor current.
By way of example, a typical silicon-based multi-phase dc/dc converter for a 120 Amp, 500 kHz microprocessor system consists of four phases. A silicon-based multi-phase dc/dc converter for 1 MHz/120 Amp microprocessor system may consist of five phases, and a silicon-based multi-phase dc/dc converter for a 2 MHz/120 Amp system may consist of seven phases.
In multi-phase dc/dc converters, current levels are often sensed in each phase as a means of feedback control within a system. This is done typically using on-resistance of a silicon switching MOSFET device or series resistance of an output inductor. The on-resistance method current sensing is problematic due to variations in processing for silicon MOSFETs devices, which becomes even more of a problem at higher current levels. Additionally, with improvements in inductor designs and fabrication methods, the series resistance current sensing technique has become less reliable because the improvements make it difficult to distinguish the inductor resistance from system noise.
Accordingly, a need exists for switching devices suitable for the increasingly stringent demands of applications such as dc power conversion. Additionally, it would beneficial for such devices to make possible accurate phase current sensing.